Injection devices based on a separate housing and replaceable cartridges have found widespread use in many areas, such as medical delivery systems, owing to the flexibility and economy contained in the possibility of providing the reusable housing with more or less advanced machinery for preparing, dosing, controlling and monitoring the injection procedure whereas the replaceable cartridge features can be limited to those necessary for safe confinement and simple expulsion of the pharmaceutical, features which furthermore may be adapted to each individual preparation type.
In more permanent set-ups, e.g. for hospital treatment situations, there are few design restrictions and the housing part can be highly sophisticated in view of motorized manipulation means, processor controlled operation and data collection as well as possible interfacing against other available instrumentation. Often the design freedom is also utilized to make the housing part compatible with one or several existing or standardized cartridge, syringe or injection device types, hereby increasing the application range for the instrument and reducing adaptation costs for the cartridge part.
For ambulatory purposes the design limitations are more severe, especially for self-contained devices without connectable support. Size and weight restrictions place limitations on the number and sophistication degree of functions possible to include. Automation as an alternative measure for increasing safety and avoiding misuse is similarly restricted by the added motorized means and operation repertoire by limited capacity of energy storage means.
Yet the devices have to meet all regular safety and precision aspects not only in the dosing step itself, which should allow for both multiple injections of equal and unchanged amounts as well as of precisely set individual doses, but also correct sequence and performance of initiating steps, such as mixing, deaeration and pre-ejection. The problems become more pronounced with pharmaceuticals having a limited storage stability requiring preparation steps immediately before use or being shear-sensitive and requiring preparation and dosing under controlled mechanical conditions. Two or multiple chamber cartridges are frequently used for medical requiring a reconstitution step, such as the dissolution of a stable lyophilized powder in one chamber with a solvent in another chamber. Besides the additional preparation step the multiple chamber cartridges pose special problems to size reduction efforts since they are typically larger than single chamber cartridges, partly because of necessary mixing space and partly because of the additional walls and mixing structures.
Although handy and portable injectors may be devised with the minimum of support features necessary to safely control all the abovesaid requirements and problems in the hands of a skilled operator, a general trend in long-term medication is to place the administration responsibility on the patient himself, also in the case of child or disabled persons, e.g. by use of pen-type injectors. A high degree of automation and control is then desirable to avoid mistakes, not only at the mere injections steps but also the critical initiation and preparation steps. Patients dependent on daily administrations also have a legitimate need for convenience and devices discrete enough to be brought around in daily life. The contradictory requirements on highly sophisticated and yet small and convenient devices are not met by known prior art suggestions.
The U.S. Pat. Nos. 5,112,317 and 5,232,459 describes typical mechanical and manually operated portable multidose devices in which size is minimized by allowing the devices to shrink in accord with medical consumption, in the first reference by a linear and in the second reference by a threaded retraction of an ampoule containing part into the device main part. In order not to cause problem during injection the dosing mechanism must necessarily operate in an intermittent manner unsuitable for further automation and in itself size an resource demanding. No special features are present to facilitate and secure preparatory or initiating steps before dosing.
The patent specifications U.S. Pat. No. 4,874,381, U.S. Pat. No. 4,968,299 and WO 96/05878 describe similar manually operated devices which provides for an initiation step in which medical reconstitution takes place in a two compartment cartridge. In order to safeguard a slow and cautious internal liquid transfer the relative motion between piston rod and cartridge is performed between threaded parts. Besides having the abovementioned general limitations to further automation, nothing facilitates safe shift of control between the reconstitution step and the dosing steps.
Injection devices with motorized and automation means are disclosed in several prior patent specifications as represented by e.g. U.S. Pat. No. 4,529,401, U.S. Pat. No. 4,833,384, U.S. Pat. No. 5,106,375, U.S. 5,139,484, EP 143,895, EP 293,958, DE 2,710,433. WO 85/02546 and WO 95/24233. Generally these devices are either only suited in permanent installations or, when suggested for mobile use, are ungainly with low cartridge to overall size ratio. They all lack cartridge preparation features with corresponding control means. The WO 93/02720 reference discloses a two compartment type cartridge system that may be used in a portable device. Aside from a special needle retraction system the reconstitution principle is entirely conventional without any utilization of automation possibilities
Accordingly there is a continuing need for portable injection devices relieving patients under self-administration from the burden of learning and controlling in a fail-safe manner every step in the administration scheme, preferably by building a high degree of automation into the devices, while still satisfying the patients legitimate desire for easy to use and discrete design. Although the present invention has a more general utility, it will mainly be described against this background.